Process for the preparation of cellulose acetate in an esterification medium of acetic acid, acetic anhydride, methylene chloride and sulfuric acid



United States Patent PROCESS FOR THE PREPARATION OF CELLULOSE ACETATE INAN ESTERIFICATION MEDIUM 0F ACETIC ACID, ACETIC ANHYDRIDE, METH- YLENECHLORIDE AND SULFURIC ACID Robert H. Wells, Union, N. J., assignor toCelanese Corporation of America, New York, N. Y., a corporation ofDelaware No Drawing. Application March 23, 1956 Serial No. 573,339

2 Claims. (Cl. 260-227) This invention relates to cellulose esters andrelates more particularly to an improved process for the manufacture oflower aliphatic acid esters of cellulose.

This application is a continuation-in-part of application Serial No.552,734, filed December 13, 1955, now abandoned.

It has previously been proposed to manufacture lower aliphatic acidesters of cellulose, such as cellulose acetate, for example, byesterifying cellulose with acetic anhydride in an acetylation mediumcontaining methylene chloride as a solvent for the cellulose acetatebeing formed and perchloric acid or sulfuric acid as an esterificationcatalyst. While perchloric acid is a highly efficient catalyst in thissystem, its use involves a number of hazards owing to the unstablenature of the acid and its salts. Sulfuric acid, on the other hand, is arelatively poor catalyst in this system so that its use requires anextended period for the completion of the esterification. As a result,when sulfuric acid is used as a catalyst, the capacity of a givenesterification vessel is reduced and the expense of manufacturing thecellulose acetate is increased.

It is an important object of this invention to provide a process for themanufacture of lower aliphatic acid esters of cellulose which will befree from the foregoing and other disadvantages.

A further object of this invention is to provide a novel V process forthe manufacture of lower aliphatic acid esters of cellulose employingmethylene chloride as a solvent and sulfuric acid as a catalyst in whichthe esterification will proceed at a rapid rate.

Other objects of this invention will be apparent from the followingdetailed description and claims.

According to the present invention, lower aliphatic acid esters ofcellulose are manufactured by esterifying cellulose with anesterification medium containing a lower aliphatic acid anhydride,methylene chloride as a solvent for the ester being formed and sulfuricacid as the esterification catalyst, and also containing a smallproportion of a lower aliphatic acid corresponding to the loweraliphatic acid anhydride. It has been found that the presence of thelower aliphatic acid in the esterification medium greatly increases theactivity of the sulfuric acid esterification catalyst so that theesterification will go to completion in a relatively short period oftime.

Cellulose from any suitable source, such as wood pulp or cotton linters,may be employed in carrying out the process of this invention. Thecellulose may advantageously be pretreated prior to the esterificationto increase its reactivity. A preferred method of carrying out suchpretreatment is to mix with each 100 parts by weight of the cellulosefrom 20 to 100 parts by weight of a lower aliphatic acid, such as formicacid, acetic acid, propionic acid, butyric acid, or mixtures of thesame.

Patented July 22, 1958 Following the pretreatment, the cellulose iscontacted with an esterification medium containing, for each parts byweight of cellulose, between 750 and 5000 parts by weight of methylenechloride, at least 2.0 or, preferably, between 2.7 and 10.0 mols oflower aliphatic acid anhydride and between 3 and 15 parts by weight ofsulfuric acid. The esterification medium also contains, for each 100parts by weight of methylene chloride, between 10 and 50 parts by weightof a lower aliphatic acid corresponding to the lower aliphatic acidanhydride. The

esterification is normally carried out at temperatures be tween 15 and45 C.

After the esterification is complete, the lower aliphatic acid ester ofcellulose formed may, if desired, be hydrolyzed or ripened to increaseits content of free hydroxyl groups and thereby alter its solubility andother characteristics. Thereafter, the said ester may be recovered fromthe esterification solution by mixing the same with a relatively largeproportion of a non-solvent diluent to precipitate the ester.Alternatively, the esterification solution may be treated to removetherefrom acids, salts and the like without simultaneously precipitatingthe lower aliphatic acid ester of cellulose.

This invention will now be described specifically in connection with themanufacture of cellulose acetate which is commercially the mostimportant lower aliphatic acid ester of cellulose at the present time.However, it may also be readily used for the manufacture of cellulosepropionate and cellulose butyrate.

The following examples are given to illustrate this invention further.

Example I There is mixed with 100 parts by weight of cotton linters, 35parts by weight of glacial acetic acid and the mixture is stirred for 20minutes at 25 C. There is then added to the cellulose 7 parts by weightof sulfuric acid. Thereafter, the pretreated cellulose is entered intoan acetylation medium containing 720 parts by weight of aceticanhydride, 3900 parts by weight of methylene chloride and 420 parts byweight of acetic acid at a temperature of 25.0 C. The acetylation iscompleted in 263 minutes as evidenced by the clearing of the solution.

When the acetylation is repeated, omitting the acetic acid from theacetylation medium and employing in its stead an equal volume ofmethylene chloride, 333 minutes are required for the acetylation to goto completion.

Example II The process of Example I is repeated, employing twice theamount of sulfuric acid catalyst. The time required to complete theacetylation is 186 minutes.

When, under these conditions, the acetic acid is omitted from theacetylation medium the time required to complete the acetylation is 235minutes.

It is understood that the foregoing detailed description is given merelyby way of illustration and that many variations may be made thereinwithout departing from the spirit of my invention.

Having described my invention what I desire to secure by Letters Patentis:

1. Process for the manufacture of cellulose acetate which comprisespretreating cellulose with, for each 100 parts by weight of cellulose, apretreating agent consisting essentially of from 20 to 100 parts byweight of acetic acid, and contacting the pretreated cellulose with anacetylation medium containing, for each 100 parts by weight ofcellulose, 2.7 to 10.0 mols of acetic anhydride,

750 to 5000 parts by weight of methylene chloride, and 3 to 15 parts byweight of sulfuric acid, and, for each 100 parts by Weight of methylenechloride, 10 to 50 parts by Weight of acetic acid the presence of thelatter acid serving to increase the esterification rate.

2. Process as set forth in claim 1 in which said pretreating is effectedwith 35 parts by weight of acetic acidper 100 parts of cellulose. j

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References Cited in the file of this patent UNITED STATES PATENTSClement Jan. 4, 1938 Fraizy Jan. 2, 1951

1. PROCESS FOR THE MANUFACTURE OF CELLULOSE ACETATE WHICH COMPRISESPRETREATING CELLULOSE WITH, FOR EACH 100 PARTS BY WEIGHT CELLULOSE, APRETREATING AGENT CONSISTING ESSENTIALLY OF FROM 20 TO 100 PARTS BYWEIGHT OF ACETIC ACID, AND CONTACTING THE PRETREATED CELLULOSE WITH ANACETYLATION MEDIUM CONTAINING, FOR EACH 100 PARTS BY WEIGHT OFCELLULOSE, 2.7 TO 10.0 MOLS OF ACETIC ANHYDRIDE, 750 TO 5000 PARTS BYWEIGHT OF METHYLENE CHLORIDE, AND 3 TO 15 PARTS BY WEIGHT OF SULFURICACID, AND, FOR EACH 100 PARTS BY WEIGHT OF METHYLENE CHLORIDE, 10 TO 50PARTS BY WEIGHT OF ACETIC ACID THE PRESENCE OF THE LATTER ACID SERVINGTO INCREASE THE ESTERIFICATION RATE.